vfio: Fix smatch errors in vfio_combine_iova_ranges()

smatch reports:

vfio_combine_iova_ranges() error: uninitialized symbol 'last'.
vfio_combine_iova_ranges() error: potentially dereferencing uninitialized 'comb_end'.
vfio_combine_iova_ranges() error: potentially dereferencing uninitialized 'comb_start'.

These errors are only reachable via invalid input, in the case of
@last when we receive an empty rb-tree or for @comb_{start,end} if the
rb-tree is empty or otherwise fails to produce a second node that
reduces the gap.  Add tests with warnings for these cases.

Reported-by: Cong Liu <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Reviewed-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Brett Creeley <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alex Williamson <[email protected]>

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